Siman 57
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א אוֹמֵר שְׁלִיחַ צִבּוּר: בָּרְכוּ אֶת ה' הַמְבֹרָךְ, וְעוֹנִין אַחֲרָיו: בָּרוּךְ ה' הַמְבֹרָךְ לְעוֹלָם וָעֶד, וְחוֹזֵר שְׁלִיחַ צִבּוּר וְאוֹמֵר: בָּרוּךְ ה' הַמְבֹרָךְ לְעוֹלָם וָעֶד: נָהֲגוּ שֶׁהַשְּׁלִיחַ צִבּוּר מַאֲרִיךְ בְּבָרְכוּ וְהַקָּהָל אוֹמְרִים יִתְבָּרַךְ וְיִשְׁתַּבַּח וְכו' בְּעוֹד שֶׁהוּא מַאֲרִיךְ בְּבָרְכוּ (טוּר).
1 Laws of Bar'khu and its responses comprising 2 sections: The prayer leader says: "Bar'khu et Adonai ha-mevorach", and they [the congregation] answer after him: "Barukh Adonai ha-mevorach l'olam va'ed", then the prayer leader returns and says: "Barukh Adonai ha-mevorach l'olam va'ed". Rema: And some have the custom that the prayer leader lengthens "Bar'khu", and the congregation say "Yitbarakh v'yishtabach..." while he is extending "Bar'khu" (Tur).
ב מָקוֹם שֶׁנָּהֲגוּ לִצְעֹק עַל חַבְרֵיהֶם בֵּין קַדִּישׁ וּבָרְכוּ לְיוֹצֵר אוֹר, אוֹ לְדַבֵּר בְּצָרְכֵי רַבִּים טוֹעִים הֵם (וְעַיֵּן לְעֵיל סוֹף סִימָן נ"ד):
2 A place where the practice is to shout at/rebuke their fellows between Kaddish/ Bar'khu and Yotzer Ohr or to speak regarding the needs of the public... they are mistaken. (See above, at the end of Siman 54)